Renal glycosuria has been thought to be mainly the result of defective glucose reabsorption in the proximal tubule. In order to evaluate the effect of reduction in proximal tubule glucose transport on urinary glucose excretion, recollection micropuncture studies were performed in 22 acutely thyroparathyroidectomized dogs following: 1) 10% extracellular volume expansion, 2) intravenous maleic acid 0.15 mmoles/kg, and 3) intravenous maleic acid 0.3 mmoles/kg. Glucose in proximal tubule fluid samples was analyzed by ultramicrofluorometric method using hexokinase. Our results showed that the proximal tubule glucose reassorption was reduced by 34, 18 and 23% of filtered loads by the three maneuvers. However, fractional glucose excretion in the urine remained unchanged after volume expansion and minimally rose after low-dose maleic acid, but increased prominently only after high-dose maleic acid. In contrast, natriuresis occurred in all three groups, suggesting dissociation of distal sodium and glucose reabsorrption as compared to their parallel proximal tubule transport. These observations indicate that the increased distal load of glucose was almost completely reabsorbed in the first two groups, whereas additional inhibition of distal glucose reabsorption by high-dose maleic acid resulted in glycosuria. It was concluded that distal glucose reabsorption plays an important role in the final regulation of urinary glucose excretion.